1. Field of the Invention
This invention relates to an electric circuit, which may be incorporated in a removable plug, for converting a signal proportional to the current in an electrical conductor to a voltage signal for use in operating the trip circuit of a circuit breaker which opens the circuit of the electrical conductor in response to an overcurrent condition. In particular, it is related to such a circuit which incorporates active electronic components powered by a signal proportional to current and which can be used to adjust the rating of the circuit breaker, to compensate for non-linearities in the transformer which generate the signal proportional to current and/or to test the circuit breaker. It can also provide an indication of an overcurrent condition prior to tripping of the breaker and an indication of sufficient current to perform the test.
1. Background Information
In some types of circuit breakers, the operating point at which the breaker trips is set by a replaceable rating plug containing resistance which is inserted into a solid state trip circuit in the circuit breaker. The trip circuit in turn controls the tripping of the breaker. The secondary current from current transformers in the breaker flows through the rating plug and the voltage across the rating plug is therefor proportional to the product of the breaker current and the rating plug resistance. If the resistance in the rating plug is made greater, the required current to produce a given voltage across the rating plug is decreased and vice versa. Because the trip circuit "sees" the rating plug voltage as a measure of the breaker current, changing the rating plug resistance effectively changes the current rating of the breaker.
A problem often arises in such circuit breakers because, at very high currents, the current transformers supplying the rating plug saturate and become non-linear. The reason for this non-linearity is the small space available inside a circuit breaker to fit current transformers. This limit on available space forces the amount of copper and iron in the transformer to be minimized, resulting in saturation at high currents and therefore, the secondary current is less than proportional to the breaker current. The result is that at high currents, the rating plug voltage is less than proportional to the breaker current. This is an undesirable situation because a greater than normal current is required to trip the breaker. A means of compensating for current transformer non-linearity would be desirable.
Some rating plugs provide an adjustable rating for the circuit breaker. One type of such rating plugs contains a relatively high power rheostat in series with a fixed resistor so that the rating plug resistance can be continuously varied over a range of values. The problem with this type of adjustable rating plug is that the high power rheostats are large and potentially unreliable. An adjustable rating plug not requiring the use of a high power rheostat would be desirable.
Another type of adjustable rating plug utilizes a number of series connected resistors and a switch for selectively inserting a desired amount of resistance into the circuit. This arrangement eliminates the bulky potentiometer but stills dissipates a large amount of power and does not provide for continuous or fine adjustments of the rating setting. Thus it would be desirable to have a rating plug which would offer the continuous adjustment of the potentiometer but without the bulk and high power consumption.
The above discussed circuit breakers with solid state trip units have been tested by supplying the trip circuit with a relatively low current from an external test source. The output from the test source simulates an overcurrent condition in the breaker causing it to trip with the appropriate delay. The problem with using a separate test source is that since its output simulates an overload condition, it is also large enough to damage delicate solid state circuitry if it persists for a sufficient length of time. When a trip occurs due to an actual overcurrent flowing through the breaker, the current supplied to the solid state trip circuitry ceases when the breaker trips and, therefore, the trip circuit is automatically protected. In a simulated overcurrent condition, using an external test source, however, the current supplied to the solid state trip circuitry is not automatically interrupted when the breaker trips. If the external test source is accidentally left on for an extended time, damage can occur to the trip circuit.
Such a test unit powered by an external source is disclosed in U.S. Pat. Nos. 3,852,660 and 3,924,160. This test unit selectively generates a lower current test signal for verifying the delayed trip, and a high level current signal which tests the instantaneous trip feature. To avoid the prolonged application of the high level signal, a timer circuit disconnects the high level signal after a predetermined interval. However, this can produce a large pulse which can be a problem. There remains therefore a need for means for testing circuit breakers which does not require separate source and which would eliminate the possibility of trip unit damage.
In some circuit breakers in which the trip circuit is powered by the current flowing through the breaker, the breaker must be carrying at least about half rated power in order for the trip circuit to operate. There is a need therefore for an indication that sufficient current is flowing through such a breaker in order to perform a test.
It is also known to provide a means indicating that a circuit breaker is experiencing an overload. However, this feature adds cost to and complication to the trip unit and may not be required or needed by all users. A simple way to add this feature to the trip unit after it is manufactured or as an option feature would be desirable.